The present invention relates to resonant sensors for determining structural property changes, and has particular though not exclusive application to such sensors for detecting the presence of chemical or biochemical species.
It is well established practice to provide resonant mass sensors in which the change in resonant frequency which occurs on the application of mass to the sensor is used to calculate the value of the applied mass.
However, such devices rely upon absolute changes in frequency to determine the added mass, the devices commonly operating at frequencies of the order of 10-100 MHz. As changes of less than 1000 Hz often need to be measured, the stability of the oscillator is of great importance, precise thermal characteristics and other associated parameters of the environment needing to be known to achieve meaningful mass measurements.
It would be desirable to be able to provide a resonant sensor more readily and directly able than heretofore to measure structural property changes, such as the addition of mass to, or the subtraction of mass from, the sensor, the measurements being independent of environmental conditions.
According to the present invention there is provided a resonant sensor for determining structural property changes, the sensor comprising a structure mounted to be capable of resonating, the structure having a cyclically symmetrical configuration with two independent degenerate modes of vibration of a common natural frequency, and means for exciting the structure to resonate according to said two degenerate modes, regions of the structure being modified such that, on changes in the structural properties of the modified regions, the natural frequencies of the two modes of vibration become different, the difference in frequencies being proportional to the change in structural properties.
Thus it will be appreciated that when, for example, a chemical or biological species is incident upon the modified regions of the surface to change the mass of said regions, the cyclic symmetry of the sensor is destroyed, and a frequency split of the previously degenerate modes is created. The value of the difference in frequencies enables the added mass to be calculated.
As any frequency change in the two modes caused by, for example, variations in temperature, pressure and internal stress in the structure are the same in each mode, these factors do not contribute to the value of the frequency split, and need not be known to determine the desired property changexe2x80x94the arrangement is thus self-compensating for these external effects, and requires only a single readout of the frequency difference to be made to enable calculation of the structural change under question.
The resonant structure may comprise a diaphragm, the periphery of which may be secured to a supporting substrate, the modified regions of the diaphragm being on the upper surface thereof, the means for exciting the diaphragm being located below the diaphragm symmetrically disposed relative to said modified regions, which means may be, for example, electrostatic, magnetic, piezoelectric or the like.
The modified regions of the diaphragm may be of, for example, gold chemically treated to promote bonding with specified chemical or biological species the presence of which is to be detected.